Method of casting magnesium and alloys thereof



Patented. May 15, 1934 PATENT.

1,959,297, FFICE METHOD OF CASTING MAGNESIUM AND annoys rrnaanor Edwin Bax-stow, Midland, Micln, assignor to The Dow Chemical-Company, Midland, Mich, a corporation of Michigan No Drawing. Application December .29, 1932, Serial No, 649,327

4 Claims. (Cl. 2 2--21 n The present invention relates to casting magnesium or an alloy thereof in which magnesium is a predominant constituent and is specifically concerned with the protection of such molten 5 metal from burning while being poured into a mold. In the following description and in the appended claims I use the term magnesium to mean not only pure magnesium, but also alloys thereof containing more than 50 per cent of maglO nesium.

Magnesium in the molten condition at temperatures ordinarily employed in foundry operations,

such as melting, pouring, and casting, has a great affinity for both oxygen and nitrogen so that on exposure to air even momentarily, as when being poured into a mold from a ladle or melting pot, there is formed on the surface of the pouring stream a more or less thin film or skin of relav tively infusible material comprising the oxide 20, and nitride of magnesium. Similar effects are produced by contact with reducible gases such as carbon dioxide, sulphur dioxide and the like. Such film or skin formation at times may be accompanied by burning of the magnesium. Inasmuch as particles of this oxidized or burnt material of the skin do not float readily on the molten metal, but are easily entrained therewith, they may be carried into the mold where they tend to impede the flow of the metal and cause cold-shuts or misruns, as well as internal flaws in the casting due to skin inclusions. The contaminated metal, furthermore, usually cannot be remelted without purification. Although it is known that oxidation or burning of the metal 36 within the mold resulting from the action of water vapor generated by the hot metal in contact with moist molding materials may be prevented by mixing an oxidation inhibitor with the molding material, so far as I am aware, no satis- 40 factory procedure is available by which oxidation or nitridation of the pouring stream outside the moldimay be prevented.

It is an object of my invention to provide a method of pouring magnesium into a mold whereby cold-shuts, misruns, and internal flaws in the casting resulting from oxidation, or nitridation outside the mold may be materially reduced. A furtherobject is to prevent the waste of metal due to contamination with oxide-nitride 5o skins produced on the pouring stream outside the mold.

,It has been'proposed heretofore to overcome somewhat similar dimculties .encountered in casting brass by burning alcohol, benzene, or light oil around the opening to the mold by which the oxygen content is reduced in the atmosphere to which the molten metal is exposed when on its way to the mold. I have found, however, that the useof this procedure with magnesium does not prevent the formation of oxide or nitride skins thereon, inasmuch as the removal of oxygen from the air surrounding the stream of flowing metal by burning such combustible materials therein produces products of combustion which are substantially as active as ordinary air in bringing about oxidation or nitridation. The Y useof a relatively dry gas such as carbon dioxide to displace the air in the mold in casting other metals does not overcome the dimculty inasmuch as carbon dioxide may be reduced by molten magnesium. Illuminating gas, the use of which has been suggested to displace the air in the mold in casting brass, is likewise ineffective to prevent I the formation of skins upon magnesium. Blanketing molten ferrous metals with the vapor of a volatile liquid such as carbon tetrachloride is ineifective with magnesium, inasmuch as this metal acts upon carbon tetrachloride reducing it to carbon with the formation of magnesium chlo'-, ride and phosgene.

It has been proposed to place sulphur directly onto the surface of molten magnesium so as to form a layer of sulphur vapor between the metal and theair. The direct application of sulphur to-the pouring stream, however, has the disadvantage that sulphur may be carried into the mold andbe trapped between the metal and the mold walls whereby blow holes or pits may be produced due to evolution of sulphur vapor be-' tween the metal and the mold. When it is attempted to burn sulphur around the path of the flowing metal so as to consume the oxygen in the ambient atmosphere not only is an objectionable concentration of sulphur dioxide produced, but also the sphere of action of the burning sulphur is largely confined to that region in w ich the sulphur is actually burning. This area is usually too small to reduce sufiiciently the oxygen content of the air adjacent to the metal. Thus burning sulphur in this manner is unsatisfactory to prevent film formation on the pouring stream. I have discovered that to overcome film or skin formation during the interval in which the metal is poured from a ladle or the like into a mold by providing means 'to modify the atmosphere in contact with the metal, it is necessary that the atmosphere contain substances which inhibit the attack of nitrogen, as well as oxygen, upon magnesium. If a combustible material is burning in the path of the molten metal whereby the free oxygen is consumed and combined oxygen that' carbon bisulphide alone may be substituted for the mixture with similar results inasmuch as it will consume the oxygen in the air and prevent the products of its combustion and residual nitrogen from attacking magnesium. It appears that some carbon bisulphide vapor which is present during combustion in the gases surrounding the metal exerts an inhibiting effect on the nitridation occasioned by air deprived of free oxygen and oxidation by the presence of carbon dioxide, or sulphur dioxide formed in the atmosphere surrounding the stream of flowing metal, so that oxide or nitride films which otherwise are liable to form on the metal, are greatly reduced.

In carrying out my invention I provide means to burn carbon bisulphide alone or in admixture with another combustible fluid miscible therewith, around the opening to the mold so that the flame substantially surrounds or encloses thestream of metal which is being poured.

In this way I bring the products of combustion of carbon bisulphide which contain air deprived of its free oxygen and unburned carbon bi sulphide vapor into close proximity with the pouring stream of molten metal on its way tothe mold.

A convenient mode of practicing the invention consists in moistening a ring-shaped wick of asbestos or the like with carbon bisulphide or a mixture of carbon bisulphide and gasoline or similar combustible fluid, placing the moistened ring around the opening to the mold and igniting the wick. When the flame has attained a suflicient height to encompass the space between the pouring spout of the ladle, crucible or the like and the mold opening, the molten metal is poured through the' atmosphere resulting therefrom into the mold.

I have found that I may use either substantially pure carbon bisulphide or mixtures thereof with gasoline or similar petroleum distillate providing not less than about 10 or 15 per cent or more of carbon bisulphide is contained therein. A suitable mixture for the purpose consists of from 10 to 50 per cent or more of carbon bisulphide and from less than 50 to 90 per cent by volume of a petroleum distillate having a boiling range between 120 F. and 300 F. Benzene (CeHs),

coal tar naphtha, alcohol or acetone or similar readily combustible fluid miscible with carbon bisulphide may be substituted for the petroleum distillate.

The following examples are illustrative of the invention and the manner in "which it may be carried out.

Example 1 In casting a plate 20 inches long, v8 inches wide and 0.125- inch thick of a magnesium alloy containing about 4 per cent of aluminum, 0.3 percent of manganese and the balance magnesium, in a temporary water-bound sand mold containing a conventional protective agent to;

prevent water vapor generated by the heat of the -cast metal from attackingv the casting in the mold, I placed an asbestos wick moistened with a mixture of 25 per cent of carbon bisulphide andper cent of gasoline around the opening to the mold, ignited the wick and then poured the casting so as to run the molten metal from the ladle through the burning atmosphere into thev mold. Out of 100 castings made in this way over were free from flaws.

, Example 2 The same shape casting was poured in a similar manner and mold to that in Example 1 except that a mixture of 25 per cent of carbon bisulphide and 75 per cent of kerosene was burned upon the wick surrounding the opening to the mold. Out of 100 castings made in this manner over 87 were free from flaws.

In comparison with these examples which illustrate the effectiveness of the method, similar castings were made, but instead of using my carbon bisulphide mixtures I burned other substances. hitherto proposed around the sprue in similar manner and obtained the following results. When gasoline alone was burned about 40 per cent of the castings were defective; similar results were obtained using benzene or kerosene; with sulphur about 35 per cent of the castings were defective; with a mixture of sulphur and gasoline about 40 per cent of the castings were defective.

These tests show that by my improved method I may greatly reduce the number of defective castings resulting from oxidation or nitridation of the pouring stream of molten magnesium by causing the same to pass through an enveloping atmosphere produced by burning carbon bisulphide, or combustible mixtures containing the same.

Other modes of applying the principle of my 195 invention may be employed instead of those explained, change being made as regards the method herein disclosed, provided the means stated by any of the following claims or the equivalent of such stated means be employed.

I therefore particularly point out and distinctly claim as my invention:

1. In a method of pouring a casting of magnesium or an alloy thereof so as to prevent the formation of oxide-nitride skins on the pouring 5 stream, the steps which comprise causing carbon bisulphide to burri around the opening to a mold and pouring the molten metal thereinto through the atmosphere resulting from such combustion.

2.In a method of pouring a casting of mag- 1120 nesium or an alloy thereof so as to prevent the formation of oxide-nitride skins on the pouring stream, the steps which comprise causing a mixture of carbon bisulphide and a volatile combustible fluid miscible therewith to burn around the "opening to a mold and pouring the molten metal thereinto through the atmosphere resulting from such combustion.

3. In a method of pouring a casting of magnesium or an alloy thereof so as to prevent the J80 formation of oxide-nitride skins on the pouring stream, the steps which comprise causing a mixture of from 10 to 50 per cent of carbon bisul- .phide and from 90 to 50 per cent of a volatile combustible fluid miscible therewith to burn 2135 around the opening to the mold and pouring the molten metal thereinto through the atmosphere resulting from such combustion.

4. In a method of pouring a casting of magnesium or an alloy thereof so as to prevent the combustion.

EDWIN O. BARSTOW. 

